The present invention relates to electronic devices, particularly to the fabrication of electronic devices such as transistors and solar cells, and more particularly to the fabrication of multi-terminal electronic devices using a pulsed high energy source to melt and crystallize amorphous silicon deposited on a low temperature substrate and without excessive heating of the substrate.
In recent years substantial effort has been directed toward methods and apparatus for crystallizing amorphous material, in the fabrication of semiconductor material, with a beam of energy, such as produced by a continuous wave (CW) laser. These prior efforts are exemplified by U.S. Pat. No. 4,309,225 issued January 1982 to J.C.C. Fan et al.; U.S. Pat. No. 4,372,989 issued Feb. 8, 1983 to G. Menzel; U.S. Pat. No. 4,377,031 issued Mar. 22, 1983 to H. Goto et al.; U.S. Pat. No. 4,414,242 issued Nov. 8, 1983 to T. Nishimura et al.; U.S. Pat. No. 4,510,015 issued Apr. 9, 1985 to R. J. Ellis et al.; and U.S. Pat. No. 4,529,617 issued Jul. 16, 1985 to A. Chenevas-Paule et al.
Various methods have also been developed for changing the crystalline structure of silicon for use in semiconductors. These methods have utilized a seed layer, such as a layer of silicon dioxide (SiO.sub.2) between the substrate, generally composed of silicon, and the amorphous silicon layer, for example, which is converted to polycrystalline silicon, for example, by application of electron beam or laser energy as produced by a CW laser, to the amorphous silicon layer. These development efforts are exemplified by U.S. Pat. No. 4,319,954 issued Mar. 16, 1982 to L. K. White et al.; U.S. Pat. No. 4,448,632 issued May 15, 1984 to Y. Abasaka; U.S. Pat. No. 4,649,624 issued Mar. 17, 1987 to R. E. Reedy; U.S. Pat. No. 4,655,850 issued Apr. 7, 1987 to S. Kakimoto et al.; U.S. Pat. No. 4,714,684 issued Dec. 22, 1987 to K. Sugahara et al.; and U.S. Pat. No. 4,751,193 issued Jun. 14, 1988 to J. J. Myrick.
More recent efforts have been directed to converting the amorphous silicon deposited on a silicon substrate to another phase and doping thereof using pulsed laser energy which pulses are long enough to melt the amorphous silicon and a thin layer of the silicon substrate beneath it, but short enough to prevent thermal damage to the silicon substrate and/or to material deposited on the opposite side of the silicon substrate. Such an approach is exemplified by U.S. Pat. No. 4,824,489 issued Apr. 25, 1989 to G. W. Cogan et al.
Pulsed laser processing (PLP) typically entails applying short (.about.35 ns) pulses of laser energy (0.1-1 Jcm.sup.-2) to silicon or thin films of silicon, in vacuum or a dopant atmosphere. Gas-immersion laser doping (GILD) has been extensively developed for use in making shallow, highly doped (&gt;10.sup.20 cm.sup.-3) and activated semiconductor junctions. See K. H. Weiner et al., IEEE Electron Device Lett. 13, 369 (1992). Others have used laser processing to achieve low temperature processing for use in making thin film transistors. See K. Sera et al., IEEE Electron Device Lett. 36, 2868 (1989), and R. Z. Bachrach et al., J. Electron. Mater. 19, 241 (1990).
In these prior developments and research efforts to improve crystallinity of amorphous material, substrates of silicon or the so-called high temperature plastic or polymeric materials, such as polyimide (e.g. KAPTON, made by DuPont Corp.) have been used, which are capable of withstanding sustained processing temperatures of higher than about 200.degree. C. For example, sustained processing temperatures of 800.degree.-900.degree. C. have been used with silicon substrates and processing temperatures of 400.degree.-450.degree. have been used with substrates of the high temperature plastic or polymeric materials.
The high temperature plastics are more expensive, have lower resistance to ultra-violet light, lower strength, and less transparency compared to commercial grade, transparent plastics referred to as low temperature plastics, which are incapable of withstanding sustained processing temperatures of higher than about 180.degree.-200.degree. C. Sustained processing is defined as a time period of longer than about 10.sup.5 ns.
It has been recognized by the inventors of the present invention that a substantial increase in efficiency, with reduced weight and cost would result in the fabrication of multi-terminal electronic devices, such as transistors and solar cells, using substrates of the so-called low temperature plastics and pulsed laser processing for crystallization and doping of amorphous silicon deposited on such low-temperature plastic. Thus, the present invention enables the use of inexpensive low-temperature, commercial grade plastics as substrates in the fabrication of solar cells, for example, whereby crystallization and doping of amorphous material deposited on such substrates could be carried out by using pulsed energy sources without heating the substrate to a temperature greater than about 180.degree.-200.degree. C. for a time period longer than about 10.sup.5 nanoseconds, thereby preventing thermal damage to the low temperature substrates.